The study of the organic light emitting device (OLED) was begun by A. Bernanose et al. at 1950s. The initially researched material is anthracene single crystal. The required driving voltage is very high (several hundreds volts) since the single crystal is too thick (10-20 μm). An anthracene thin film was made by Vinceet in 1982 via using vacuum evaporation, blue fluorescence was observed under the voltage of 30V, but its external quantum efficiency was only 0.03%. The early organic light-emitting device was high in voltage, low in luminance and efficiency, and thus at a low level. An organic small molecule light-emitting device having a structure of ITO/Diamine/Alq3/Mg:Ag was reported by C. W. Tang and Vanslyke of Eastman Kodak of the USA in 1987. As to the above device, when it was operating under a voltage of 10V, the luminance was up to 1000 cd/m2 and the external quantum efficiency was up to 1.0%, therefore, it had attracted a broad attention of scientists. Subsequently, the news that electroluminescence could also be achieved by using conjugated polymer PPV was firstly reported in Nature by J. H. Burroughes et al in 1990, they fabricated a polymer organic light-emitting device having a structure of ITO/PPV/Ca by spin-on, and the external quantum efficiency of this device was 0.05%. The people saw the possibility of applying the organic light-emitting device into a display from the report above. Then, the curtain was rung up on the research and industrialization of the organic small molecule and the polymer electroluminescence.
High efficiency, long lifetime, and high brightness etc. of the OLED device are the foundation, based on which the industrialization of the OLED device can be realized. The mobility of an electron is lower than that of a hole in general organic material, so it is very important to improve the electron injection efficiency. The barrier of injection of electrons into an organic layer can be decreased by using a metal having low work function (for example, Ca, Mg and so on), and thus the electron injection can be increased. Another method for improving the electron injection is to add a layer of an inorganic compound between a cathode layer and an anode layer. It is demonstrated by practice that LiF/Al is a cathode structure with excellent electron injection ability and is widely used in OLED products. However, the light emitting quenching occurs due to the present of halogen atoms, the material has high toxicity, the temperature at which this material is transformed into a film is high, and there are stick demands on the film thickness of the electron injection layer. When LiAlO2 and Li2CO3 are used as the injection layer material, the lifetime is short and the efficiency is low. Many kinds of materials were proposed by people as the electron injection material at an early stage. However, the scheme, where a compound containing two kinds of elements of Group IA is employed, has not been found as yet.